1. Technical Field of the Invention
The present invention relates to a method for coating a sliding surface of a high-temperature member, the high-temperature member, and an electrode for an electro-discharge surface treatment.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a shrouded turbine blade as an example of a high-temperature member seen from a shroud portion, and FIG. 2 is another perspective view of a shroud section.
The shrouded turbine blade is constituted of a blade portion 1, a dove tail portion 2, and a shroud portion 3. The blade portion 1 is a portion which has a blade-shaped section and in which a rotation power is generated by combustion gas. The dove tail portion 2 is positioned in a terminal end portion of the blade portion 1, and is fixed to a turbine disk (not shown) to transmit the rotation power to the turbine disk. The shroud portion 3 is attached to a blade tip end, and has a function of suppressing vibration or reducing a gas leakage in the tip end.
As shown in this figure, the shroud portions are formed to be integrated with one or a plurality of blade portions 1, and are assembled to closely contact with one another. The closely contact surface (abutment surface) is simply linear as seen from a radial direction in some case, and is stepped midway in the other case as in this example. The surface stepped midway will hereinafter be referred to as “Z-notched”.
For a Z-notched shroud portion, shroud portions disposed adjacent to each other as shown by a two-dot chain line in FIG. 1 abut on each other on side surfaces A, B of a Z notch 3a to hold the entire position. Therefore, the Z-notched shroud portions have characteristics that the portions have a high position hold capability without being connected to each other.
However, the turbine blade rotate at a high speed during operation, and not only undergoes periodic deformation and vibration but also is exposed to the combustion gas at a high temperature passed through the turbine blade. Therefore, there is a problem that the side surfaces A, B of the shroud portion receive a high surface pressure at the high temperature while rubbing, and are severely abraded. It is to be noted that the sliding surfaces A, B are not fixed, and one surface is defined as A while the surface abutting on the one surface is defined as B.
To solve the problem, a hard heat-resistant/wear-resistant metal has heretofore been build-up welded or thermally sprayed to the surfaces disposed adjacent to the Z notch of the turbine blade and sliding against each other (side surfaces A, B, hereinafter referred to as “sliding surfaces A, B”) (e.g., [Reference 1]).
[Reference 1]
Japanese Laid-Open Patent Publication No. 2001-152803
However, for the build-up welding and thermally spraying, a layer forming rate is high, but there are problems that soundness/adhesion of layers, dimensional precision, and workability are bad, and automation is difficult. There are also problems that a pre-treatment/post-treatment are necessary and cost is high.
On the other hand, as shown in FIGS. 3 and 4, a large number of fitting portions 5 are used in a turbine high-temperature section. As shown in FIG. 5, a large number of sliding surfaces exist even in a stator blade segment of a compressor. A rear stage of the compressor has a high temperature during the operation.
The fitting portions of the turbine high-temperature section and the sliding surfaces of the compressor rub against each other during the operation and are therefore easily worn, and need to be coated with a high-temperature wear-resistant material. Therefore, a hard material has heretofore been attached to the surface by welding or thermal spraying. That is, a surface layer of a component is activated with grinding or blast before the welding or the thermal spraying. Subsequently, a Stellite-based alloy is built up by the welding or the thermal spraying, and grinding processing is carried out to remove an excess metal and to secure dimensions after the welding or the thermal spraying. However, there are problems that the fitting portions are narrow trenches, the welding or the thermal spraying is not easily carried out, and the portions of conventional materials are easily worn.
On the other hand, a technique for coating the surface of the metal material by in-liquid electro-discharge to impart corrosion and wear resistances has already been applied for a patent and well known. Main points of this technique are as follows. First, powders of tungsten carbide (WC) and Co are mixed, compressed, and molded into an electrode, the in-liquid electro-discharge is carried out with the electrode, and an electrode material is deposited on a work. Thereafter, re-melting electro-discharge processing is carried out on the electrode material deposited on the work by another electrode (e.g., a copper electrode, graphite electrode, and the like), and a higher hardness and adhesion are obtained in this method. This related art will hereinafter be described.
First, a second related art will be described (e.g., refer to Reference 2). A mixed green compact electrode of WC—Co is used to perform the electro-discharge processing on the work (mother material S50C) in the liquid and to deposit WC—Co (primary processing). Subsequently, re-melting processing (secondary processing) is carried out by an electrode, which is not worn out very much such as a copper electrode. Only with the deposition of the primary processing, a coat layer has a hardness (Vickers hardness) of about Hv=1410 and includes a structure in which there are many hollows. However, by the secondary processing which is the re-melting processing, the hollows are eliminated, and a coat layer having an improved structure indicating a hardness of Hv=1750 is obtained. In accordance with the method of the second related art, a hard coat layer having good adhesion is obtained with respect to a steel material. However, it is difficult to form the coat layer having a strong adhesion on the surface of a sintered material such as a sintered hard alloy.
Next, in accordance with a third related art by the present applicant, it has been clarified that the material forming hard carbide such as Ti is used as the electrode to generate the electro-discharge between the electrode and a metal material constituting the work, and then a strong hard coating layer can be formed on the surface of the metal as the work without any re-melting process (e.g., see Reference 3). This is because the electrode material worn by the electro-discharge reacts with carbon C that is a component in dielectric liquid to generate titanium carbide (TiC). Furthermore, it has been found that when the electro-discharge is generated between the metal material of the work and the green compact electrode of hydride of the metal such as titanium hydride (TiH2), the hard coating layer having good adhesion can be formed more quickly as compared with the use of the material such as Ti. It has further been found that when the electro-discharge is generated between the metal material of the work and the green compact electrode obtained by mixing hydride such as TiH2 with another metal or ceramic, a hard coating layer having various properties such as hardness and wear resistance can quickly be formed.
Moreover, in accordance with a fourth related art, it has been found that a surface treatment electrode having a high strength can be prepared by preliminary sintering (e.g., see Reference 4). As one example of the fourth related art, the preparation of the electrode for the electro-discharge surface treatment, constituted of a mixed powder of WC and Co powders will be described. For the green compact obtained by mixing, compressing, and molding the WC and Co powders, the WC and Co powders may simply be mixed, compressed, and molded. However, when the powders are compressed/molded after mixing wax, moldability of the green compact is more preferably improved. However, the wax is an insulating material. Therefore, when a large amount of wax is left in the electrode, electric resistance of the electrode increases, and therefore electro-discharge properties are degraded. Therefore, it is necessary to remove the wax. When the green compact electrode is charged and heated in a vacuum furnace, the wax can be removed. At this time, when the heating temperature is excessively low, the wax cannot be removed. When the temperature is excessively high, the wax turns to soot, and purity of the electrode is degraded. Therefore, the heating temperature needs to be kept at a temperature which is not less than a temperature to melt the wax and which is not more than a temperature to decompose the wax and to turn to soot. Next, the green compact in the vacuum furnace is heated by a high-frequency coil and calcined to such an extent that a strength can bear machining, such that the compact is not excessively hardened, and exhibits a hardness like that of chalk. However, a contact portion between carbides is calcined at a comparatively low sintering temperature such that coupling is advanced but weak coupling is achieved before real sintering. It is known that when the electro-discharge surface treatment is carried out with this electrode, a dense and homogeneous coat can be formed.
Next, in accordance with a fifth related art, it has partially been seen that various functions can be imparted to the coating layer by adjustment of materials to be blended as the electrode materials (e.g., see Reference 5). In this fifth related art, it is disclosed that lubricating properties can be imparted to the coating layer by mixture of a material indicating the lubricating properties with the electrode.
Moreover, in a method disclosed in a sixth related art, the work is used as a cathode, any of W or WC, Stellite-based alloy, TiB2 (titanium boride), and Cr3C2 (chromium carbide) is formed in a rod-shaped electrode to carry out the electro-discharge processing, and fixing layers such as W or WC, Stellite-based alloy, TiB2, and Cr3C2 are formed on the surface of the work. Thereafter, W or WC, Cr3C2, Co, Cr, Al, Y, and the like are thermally sprayed to the surface of the fixing layer, and thereafter the surface is further subjected to plasma processing to obtain the wear resistance (e.g., see Reference 6).
Next, in a seventh related art, an electrode for the electro-discharge processing, formed of Cr3C2 and the like to inhibit the wear on the electrode is disclosed (e.g., see Reference 7). In a method disclosed in an eighth related art, the materials such as WC, TaC, TiC, cBN (cubic boron nitride), and diamond are used as the electrode material to attach the electrode material melted by the electro-discharge in the atmosphere to the work, and the coating layer is formed (e.g., see Reference 8). Furthermore, a ninth related art relates to an electro-discharge surface treatment technique, and lubricating materials such as BN are added to the electrode for the electro-discharge surface treatment to impart a lubricating function to the coating layer (e.g., see Reference 9).
[Reference 2]
Japanese Laid-Open Patent Publication No. 5-148615 (pages 3 to 5)
[Reference 3]
Japanese Laid-Open Patent Publication No. 9-192937 (page 9)
[Reference 4]
WO99/58744 (pages 18 to 20)
[Reference 5]
WO00/29157 (pages 6 to 7)
[Reference 6]
Japanese Laid-Open Patent Publication No. 8-81756 (pages 2 to 3)
[Reference 7]
Japanese Laid-Open Patent Publication No. 3-66520 (page 2)
[Reference 8]
Japanese Laid-Open Patent Publication No. 8-53777 (page 3)
[Reference 9]
Japanese Laid-Open Patent Publication No. 2001-279465 (pages 4 to 5)
However, in some of the electro-discharge surface treatments disclosed in the above-described second to ninth related arts, the lubricating materials are added to form a functional coating layer, but in most of the arts, the wear resistance at room temperature is focused on, and therefore the coating layer of the hard material such as TiC is formed on the surface of a material to be processed.
On the other hand, in recent years, there has been a strong demand for a coating layer that has wear resisting properties under a high-temperature environment or lubricating properties. FIG. 6 shows a schematic diagram of a turbine blade of a gas turbine engine for an airplane. In this gas turbine engine for the airplane, as shown in the figure, a plurality of turbine blades 201 contact one another and are fixed, and are constituted to rotate around an axis. When the turbine blades 201 rotate during the operation of the gas turbine engine for the airplane, a contact portion A among the shown turbine blades 201 is extremely rubbed and hit under a high-temperature environment. There has been a problem that the wear-resistant coating layer used in the above-described related arts is degraded in hardness or oxidized under the high-temperature environment (700° C. or more) under which the turbine blade is used, and therefore there is hardly wear-resistant effect. Moreover, the coating layer to which the lubricating properties are imparted by the fourth and eighth related arts is based on an assumption of the use at room temperature. The lubricating properties at room temperature is far different in phenomenon and mechanism from those under the high-temperature environment exceeding 700° C. for use in the gas turbine engine for the airplane. These related arts have a problem that the lubricating properties in the high-temperature environment are not considered.